Integrated circuit device to support inductive sensing

ABSTRACT

An integrated circuit device inductive touch analog front end (AFE) excites selected ones of a plurality of inductive touch sensors, measures voltages across the coils of the plurality of inductive touch sensors, and provides analog output signals representative of these coil voltages. A physical displacement (touch) to the inductive sensor causes the inductance value of the inductive touch sensor to change with a corresponding change in a voltage across the coil of the inductive touch sensor. A digital processor controls selection of each one of the plurality of inductive touch sensors and receives the respective analog output voltage signal from the inductive touch AFE. When a sufficient change in the coil voltage is determined by the digital processor, that inductive touch sensor is assumed to have been actuated and the digital processor takes action based upon which one of the plurality of inductive touch sensors was actuated (touched).

RELATED PATENT APPLICATIONS

This application claims priority to commonly owned U.S. ProvisionalPatent Application Ser. No. 61/104,012; filed Oct. 9, 2008, and isrelated to commonly owned U.S. patent application Ser. No. 12/560,906;filed Sep. 16, 2009; and both are hereby incorporated by referenceherein for all purposes.

TECHNICAL FIELD

The present disclosure relates to integrated circuits, and moreparticularly to, an integrated circuit device that supports inductivesensing.

BACKGROUND

Capacitive sensors, e.g., touch sensors, are widely used as userinterfaces for a wide variety of consumer, commercial, industrial andmilitary applications. However, capacitive touch sensors suffer fromseveral shortcomings such as sensitivity to spilled liquids andunreliable operation when a user is wearing heavy gloves. Inductivetouch sensors solve the shortcomings of capacitive touch sensors andhave started to replace them in certain specialized applications notcompletely suited for the capacitive touch sensors. Inductive touchsensors require appropriate specialized interface circuits when used inan electronic system. Present technology inductive sensor interfacecircuits require a significant number of external discrete components tooperate. These external discrete components are expensive and require alarge amount of circuit board area for use in a system application.

SUMMARY

Therefore there is a need for an integrated circuit device that supportsdetection of the actuation of inductive sensors and provides usefuloutputs therefrom. The inductive sensor may be used to sense a touchcausing an inductive change in the sensor. The integrated circuitinterface that supports the operation of the inductive sensor will havecontrol mechanisms that substantially eliminate the need for externalcomponents, thus reducing physical size and costs of manufacture forsystems using inductive sensors.

According to a specific example embodiment of this disclosure, anintegrated circuit device configured as an analog front end forsupporting inductive touch sensing comprises: a voltage reference; asynchronous detector having a first input coupled to an inductivereference coil external connection, wherein the inductive reference coilexternal connection is adapted for coupling to an inductive referencecoil; a second input coupled to an inductive touch sensor coil externalconnection, wherein the inductive touch sensor coil external connectionis adapted for coupling to at least one inductive touch sensor coil; athird input coupled to a reference select external connection, whereinthe reference select external connection is adapted for coupling to areference select signal; a fourth input coupled to a clock externalconnection, wherein the clock external connection is adapted forcoupling to a clock signal, and a fifth input coupled to the voltagereference; a coil driver having an output coupled to a coil drive outputexternal connection, an input coupled to a coil drive input externalconnection; and an amplifier configured with a low-pass filter andhaving inputs coupled to the synchronous detector and an output havingvoltage values representative of inductance values of the at least oneinductive touch sensor coil and the inductive reference coil, the outputof the amplifier is coupled to a voltage detector output externalconnection; wherein the synchronous detector mixes the clock signal witha signal from the inductive reference coil or the at least one inductivetouch sensor coil, as selected by the reference select signal, toproduce sum and difference mixing products, whereby the amplifieramplifies the sum and difference mixing products and the low-pass filtersubstantially attenuates the sum mixing product and passes thedifference mixing product to the output of the low-pass filter.

According to another specific example embodiment of this disclosure, anelectronic system having an inductive touch interface comprises: aninductive touch interface comprising a plurality of inductive touchsensor coils and an inductive reference coil; a first integrated circuitdigital processor; a second integrated circuit inductive touch analogfront end comprising: a voltage reference; a synchronous detector havinga first input coupled to the inductive reference coil; a second inputcoupled to the plurality of inductive touch sensor coils; a third inputcoupled to a reference select signal from the digital processor; afourth input coupled to a clock signal from the digital processor, and afifth input coupled to the voltage reference; a coil driver having anoutput coupled in series with the inductive reference coil and selectedones of the plurality of inductive touch sensor coils, an input coupledthrough an external low-pass filter to a clock output from the digitalprocessor; and an amplifier configured with a low-pass filter and havinginputs coupled to the synchronous detector and an output having voltagevalues representative of inductance values of the plurality of inductivetouch sensor coils and the inductive reference coil, the output of theamplifier is coupled to an analog input of the digital processor;wherein the synchronous detector mixes the clock signal with a signalfrom the inductive reference coil or the at least one inductive touchsensor coil, as selected by the reference select signal, to produce sumand difference mixing products, whereby the amplifier amplifies the sumand difference mixing products and passes the difference mixing productat the output of the amplifier.

The synchronous detector may comprise: a decoder having a first inputcoupled to the clock external connection and a second input coupled tothe reference select external connection; and six analog pass-gatescontrolled by the decoder, wherein inputs of first and fourth analogpass-gates are coupled to the voltage reference, inputs of second andfifth analog pass-gates are coupled to the plurality of inductive touchsensor coils, inputs of third and sixth analog pass-gates are coupled tothe inductive reference coil; whereby the third and fourth analogpass-gates are closed when the reference select and clock signals are atfirst logic levels, the first and sixth analog pass-gates are closedwhen the reference select signal is at the first logic level and theclock signal is at a second logic level, the second and fourth analogpass-gates are closed when the reference select signal is at the secondlogic level and the clock signal is at the first logic level, and thefirst and fifth analog pass-gates are closed when the reference selectand clock signals are at the second logic level.

The synchronous detector may also comprise: a frequency mixer; a decoderhaving a first input coupled to the clock and a second input coupled tothe reference; and a plurality of multiplexers for selectively couplingthe voltage reference, the inductive touch sensor, and the inductivereference coil external connection to the frequency mixer.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present disclosure thereof may beacquired by referring to the following description taken in conjunctionwith the accompanying drawings wherein:

FIG. 1 is a schematic block diagram of an electronic system having aninductive touch keypad, an inductive touch analog front end and adigital processor, according to a specific example embodiment of thisdisclosure;

FIG. 2 is a more detailed schematic block diagram of the inductive touchanalog front end shown in FIG. 1;

FIG. 3 is a more detailed schematic block diagram of a synchronousdetector and the circuit functions shown in FIG. 2, according to aspecific example embodiment of this disclosure;

FIG. 4 is a more detailed schematic block diagram of a synchronousdetector and the circuit functions shown in FIG. 2, according to anotherspecific example embodiment of this disclosure;

FIG. 5 is a more detailed schematic block diagram of the amplifier andlow-pass filter shown in FIG. 3, according to a specific exampleembodiment of this disclosure; and

FIG. 6 is a more detailed schematic block diagram of the amplifier andlow-pass filter shown in FIG. 4, according to another specific exampleembodiment of this disclosure.

While the present disclosure is susceptible to various modifications andalternative forms, specific example embodiments thereof have been shownin the drawings and are herein described in detail. It should beunderstood, however, that the description herein of specific exampleembodiments is not intended to limit the disclosure to the particularfoil is disclosed herein, but on the contrary, this disclosure is tocover all modifications and equivalents as defined by the appendedclaims.

DETAILED DESCRIPTION

Referring now to the drawings, the details of an example embodiment isschematically illustrated. Like elements in the drawings will berepresented by like numbers, and similar elements will be represented bylike numbers with a different lower case letter suffix.

Referring to FIG. 1, depicted is a schematic block diagram of anelectronic system having an inductive touch keypad, an inductive touchanalog front end and a digital processor, according to a specificexample embodiment of this disclosure. A digital processor 106, e.g., amicroprocessor, microcomputer, digital signal processor (DSP),application specific integrated circuit (ASIC), programmable logic array(PLA), etC., is coupled to an inductive touch analog front end (AFE) 104and a matrix of inductive touch sensors 102. Preferred inductive touchsensors 102 are Microchip inductive mTouch™ sensors more fully describedat www.microchip.com.

The inductive touch AFE 104 facilitates, with a single low-costintegrated circuit device, all active functions used in determining whenthere is actuation of inductive sensors, e.g., by pressing anddeflecting a target key that changes the inductance value of anassociated inductive sensor. The inductive touch AFE 104 measures theinductance value of each key of the matrix of inductive touch sensors102 and converts the inductance values into respective analog directcurrent (dc) voltages that are read and converted into digital values bythe digital processor 106. A reference inductor (e.g., coil) (FIGS. 2and 3) may also be included in the matrix of inductive touch sensors 102for use as a comparison reference between an un-activated inductivesensor (coil) and an activated inductive sensor (coil), as more fullydescribed hereinafter.

The digital processor 106 supplies clock and control functions to theinductive touch AFE 104, reads the analog voltage detector output of theinductive touch AFE 104, and selects each key of the matrix of inductivetouch sensors 102 and the reference inductive sensor for processing bythe inductive touch AFE 104, as more fully described hereinafter. Whenactuation of a key of the matrix of inductive touch sensors 102 isdetermined, the digital processor 106 will take an appropriate action.

Referring to FIG. 2, depicted is a more detailed schematic block diagramof the inductive touch analog front end shown in FIG. 1. The inductivetouch AFE 104 comprises a synchronous detector 212, a control module214, a coil driver 210, a voltage reference 220, and anamplifier/low-pass filter 216. The synchronous detector 212 is used toextract signals from excitation of each touch sensor coil. Use of asynchronous detector (e.g., mixer) improves the signal-to-noise ratio ofthe detection process so as to produce useful or desired signals (utilesignals) for use by the digital processor 106. As explained more fullyhereinafter, the alternating current (AC) voltage amplitude from eachtouch sensor coil is mixed with a clock signal to produce sum anddifference frequencies of the two AC signals. Since the AC voltageamplitude from each inductive touch sensor coil is at the same frequencyas the clock signal, there will be a direct current (DC) voltagecomponent (difference frequency) and twice the clock signal frequency(sum frequency) signals as mixing products from the synchronous detector212.

The amplifier/low-pass filter 216 is used as a buffer-amplifier/low passfilter between the synchronous detector 212 and the V_(DETOUT) node 236.The amplifier/low-pass filter 216 functions as an integrator and passesthe DC voltage (difference frequency mixing result) while effectivelysuppressing the twice clock frequency (sum frequency mixing result). Atthe output of the amplifier/low-pass filter 216 a DC voltage isavailable to the digital processor 106 that is proportional to theinductance value of a selected one of the plurality of inductive sensors242 or the reference inductor 240, as more fully described hereinafter.The digital processor 106 converts the analog DC voltage from theinductive touch AFE 104 to a digital voltage representation thereof andassociates that digital voltage representation with the selectedinductive sensor 242 or reference inductor 240. Since the digitalprocessor 106 selects the inductive sensor 242 or the reference inductor240, matching of the DC voltage values to associated ones of theplurality of inductive sensors 242 and reference inductor 240 are easilymade.

The voltage reference 220 may be set to have a voltage output ofapproximately one-half the supply voltage, and may be an operationalamplifier configured with unity gain and a non-inverting input to aresister ladder voltage divider. Other types of voltage references maybe used effectively so long as there is adequate voltage stability andsufficient low impedance drive capability.

The coil driver 210 receives a signal derived from the clock supplied bythe digital processor 106 or from any other clock source available. Alow-pass filter comprising capacitor 252 and resistor 250 attenuate thehigher frequency components of the clock square wave signal so as toproduce approximately a sinusoidal waveform that is input to the coildriver 210, amplified, and then made available to the plurality ofinductive sensors 242 and the reference inductor 240 through a seriesconnected resistor 244. Each of the plurality of inductive sensors 242is selected by the digital processor 106 by connecting one end of theselected one of the plurality of inductive sensors 242 to a supplycommon, thereby completing the circuit from the coil driver 210 andproducing a voltage across the selected one of the plurality ofinductive sensors 242 from the AC current supplied by the coil driver210. The synchronous detector 212 detects the voltage from each one ofthe plurality of inductive sensors 242 and the reference inductor 240for subsequent processing by the digital processor 106. The coil driver210 supplies current at the clock frequency to the reference coil 240and the selected one of the plurality of inductive sensors 242 connectedin series. When the approximately sinusoidal current is flowing throughthe reference coil 240 and the selected one of the plurality ofinductive sensors 242, voltages proportional to the inductances arethereby generated.

Referring to FIG. 3, depicted is a more detailed schematic block diagramof a synchronous detector and the circuit functions shown in FIG. 2,according to a specific example embodiment of this disclosure. Thesynchronous detector 212 a comprises a decoder 360 and a plurality ofanalog pass-gates 362. The decoder 360 receives a clock (C_(LK)) signalat external connection 230 and an input selection (R_(EFSEL)) atexternal connection 228, both from the digital processor 106. Thedecoder 360 controls the on and off states of the plurality of analogpass-gates 362, as more fully described hereinafter.

The coil driver 210 generates AC voltages across the reference inductor240 and the selected one of the plurality of inductive sensors 242proportional to the inductances thereof. The selected one of theplurality of inductive sensor voltages (L_(BTN)) and the referenceinductor voltage (L_(REF)) are coupled through DC blocking capacitors246 and 248 to inputs of the synchronous detector 212 a through externalconnections 224 and 226, respectively.

The plurality of analog pass-gates 362 may operate according to thefollowing table:

REFSEL CLK Tx1 Tx2 Tx3 Tx4 Tx5 Tx6 Remarks 0 0 open open close closeopen open LREF selected 0 1 close open open open open close LREFselected 1 0 open close open close open open LBTN selected 1 1 closeopen open open close open LBTN selected

The synchronous detector 212 a has three signal inputs used formeasurement of the inductance of the inductive touch sensors, thereference inductor voltage (L_(REF)), the selected one of the pluralityof inductive sensor voltage (L_(BTN)) and the reference voltage(V_(REF)) from the voltage reference 220. The synchronous detector 212 a(mixer) can mix between two of the these three inputs at any given timeat the frequency provided at the clock connection 230 (C_(LK)). Forexample, if R_(EFSEL) is at a logic zero, then the synchronous detector212 a mixes the reference inductor voltage and reference voltagesignals. If the R_(EFSEL) is at a logic one, then the synchronousdetector 212 a mixes the selected one of the plurality of inductivesensor voltage (L_(BTN)) and reference voltage (L_(REF)) signals. Byalternately mixing the reference inductor voltage or the selected one ofthe plurality of inductive sensor voltages (L_(BTN)) with the referencevoltage (L_(REF)) at the same frequency as the approximately sinusoidalvoltage being produced by the coil driver 210, a DC signal and an ACsignal are generated at the output of the synchronous detector 212 a(mixer) that is applied to the differential inputs of differentialamplifier/low-pass filter 216 a.

The differential amplifier/low-pass filter 216 a is used as abuffer-amplifier/low pass filter between the synchronous detector 212 aand the V_(DETOUT) node 236. The amplifier/low-pass filter 216 afunctions as an integrator and passes the DC voltage (differencefrequency mixing result) while effectively suppressing the twice clockfrequency (sum frequency mixing result). This DC voltage represents theinductance of the measured reference or selected touch sensor inductor,as discussed more fully above. The DC voltage may be fed to ananalog-to-digital converter (ADC) (not shown) that is part of thedigital processor 106, whereby the digital processor 106 samples andperforms inductive touch calculations in determining when a touch sensoris actuated.

Also by alternating the polarity of connecting the touch inductor orreference inductor signals frequency mixing occurs that produces the sumand the difference frequencies between the clock input frequency and thefrequency of the coil voltage. Since both frequencies are the same (theoutput of the coil driver 210 is derived from the clock signal input)the mixing product sum of the frequencies will be twice the clockfrequency and the difference of the frequencies will be at zerofrequency, a DC voltage that is proportional to the inductance value ofthe measured coil. The differential outputs from the closed ones of theplurality of analog pass-gates 362 are applied to the differentialinputs of the amplifier/low-pass filter 216 a. The voltage reference 220DC biases the circuits of the synchronous detector 212 a at aboutone-half the operating voltage for optimal operation of theamplifier/low-pass filter 216 a. The amplifier/low-pass filter 216 aconverts the differential output from the synchronous detector 212 a toa single-ended voltage output, whereby DC utile (useful, desired)signals are made available to an analog input of the digital processor106. An isolate signal may be applied at node 250 to turn off all of theplurality of analog pass-gates 362 so as to isolate theamplifier/low-pass filter 216 a from the plurality of inductive sensors242 and the inductive sensor 240 during a Vref measurement at node 252,otherwise during normal operation the synchronous detector 212 afunctions as described hereinabove.

Referring to FIG. 4, depicted is a more detailed schematic block diagramof a synchronous detector and the circuit functions shown in FIG. 2,according to another specific example embodiment of this disclosure. Thesynchronous detector 212 b comprises multiplexers 440, 442 and 444, anda frequency mixer 446. Operation of this embodiment of the synchronousdetector 212 b is similar to the operation of the synchronous detector212 a shown in FIG. 3, and described hereinabove.

Referring to FIG. 5, depicted is a more detailed schematic block diagramof the amplifier/low-pass filter shown in FIG. 3, according to aspecific example embodiment of this disclosure. The amplifier/low-passfilter 216 may comprise an amplifier 550 having differential inputs,e.g., differential input operational amplifier; feedback resistors 258and 260, and capacitors 262 and 264 arranged in a low-pass filterconfiguration. It is contemplated and within the scope of thisdisclosure that other amplifier and low pass filter configurations maybe used for the amplifier/low-pass filter 216, as would be know to oneof ordinary skill in analog circuit design and having the benefit ofthis disclosure.

Referring to FIG. 6, depicted is a more detailed schematic block diagramof the amplifier and low pass filter shown in FIG. 4, according to aspecific example embodiment of this disclosure. The amplifier/low-passfilter 216 b may comprise an amplifier 550 having differential inputs,e.g., differential input operational amplifier; feedback resistor 260,and capacitor 264 arranged in a low-pass filter configuration. It iscontemplated and within the scope of this disclosure that otheramplifier and low pass filter configurations may be used for theamplifier/low-pass filter 216 b, as would be know to one of ordinaryskill in analog circuit design and having the benefit of thisdisclosure.

While embodiments of this disclosure have been depicted, described, andare defined by reference to example embodiments of the disclosure, suchreferences do not imply a limitation on the disclosure, and no suchlimitation is to be inferred. The subject matter disclosed is capable ofconsiderable modification, alteration, and equivalents in form andfunction, as will occur to those ordinarily skilled in the pertinent artand having the benefit of this disclosure. The depicted and describedembodiments of this disclosure are examples only, and are not exhaustiveof the scope of the disclosure.

What is claimed is:
 1. An integrated circuit device configured as ananalog front end for supporting inductive touch sensing, said integratedcircuit device comprising: a voltage reference; a synchronous detectorhaving a first input coupled to an inductive reference coil externalconnection, wherein the inductive reference coil external connection isadapted for coupling to an inductive reference coil; a second inputcoupled to an inductive touch sensor coil external connection, whereinthe inductive touch sensor coil external connection is adapted forcoupling to at least one inductive touch sensor coil; a third inputcoupled to a reference select external connection, wherein the referenceselect external connection is adapted for coupling to a reference selectsignal; a fourth input coupled to a clock external connection, whereinthe clock external connection is adapted for coupling to a clock signal,and a fifth input coupled to the voltage reference; a coil driver havingan output coupled to a coil drive output external connection, an inputcoupled to a coil drive input external connection; and an amplifierconfigured with a low-pass filter and having inputs coupled to thesynchronous detector and an output having voltage values representativeof inductance values of the at least one inductive touch sensor coil andthe inductive reference coil, the output of the amplifier is coupled toa voltage detector output external connection; wherein the synchronousdetector mixes the clock signal with a signal from the inductivereference coil or the at least one inductive touch sensor coil, asselected by the reference select signal, to produce sum and differencemixing products, whereby the amplifier amplifies the sum and differencemixing products and the low-pass filter substantially attenuates the summixing product and passes the difference mixing product to the output ofthe low-pass filter.
 2. The integrated circuit device according to claim1, wherein the synchronous detector comprises: a decoder having a firstinput coupled to the clock external connection and a second inputcoupled to the reference select external connection; and six analogpass-gates controlled by the decoder, wherein inputs of first and fourthanalog pass-gates are coupled to the voltage reference, inputs of secondand fifth analog pass-gates are coupled to the inductive touch sensorexternal connection, inputs of third and sixth analog pass-gates arecoupled to the inductive reference coil external connection; whereby thethird and fourth analog pass-gates are closed when first logic levelsare on the reference select and clock external connections, the firstand sixth analog pass-gates are closed when the first logic level is onthe reference select external connection and a second logic level is onthe clock external connection, the second and fourth analog pass-gatesare closed when the second logic level is on the reference selectexternal connection and the first logic level is on the clock externalconnection, and the first and fifth analog pass-gates are closed whensecond logic levels are on the reference select and clock externalconnections.
 3. The integrated circuit device according to claim 2,wherein the first logic level is a logic zero (“0”) and the second logiclevel is a logic one (“1”).
 4. The integrated circuit device accordingto claim 2, wherein the second logic level is a logic zero (“0”) and thefirst logic level is a logic one (“1”).
 5. The integrated circuit deviceaccording to claim 2, further comprising a frequency divider incombination with the decoder, wherein the frequency divider divides aclock frequency from the clock external connection and applies thedivided clock frequency to the analog pass-gates.
 6. The integratedcircuit device according to claim 1, wherein the amplifier is adifferential input operational amplifier.
 7. The integrated circuitdevice according to claim 1, wherein a digital processor determineswhich one of the at least one inductive touch sensor coil is selectedfor voltage measurement by the synchronous detector.
 8. The integratedcircuit device according to claim 7, wherein the digital processorsupplies the clock and reference select signals.
 9. The integratedcircuit device according to claim 7, wherein the digital processorreceives the voltage values from the output of the amplifier, determineswhich one of the at least one inductive touch sensor coil is actuatedbased upon the received voltage values.
 10. The integrated circuitdevice according to claim 1, wherein the synchronous detector comprises:a frequency mixer; a decoder having a first input coupled to the clockexternal connection and a second input coupled to the reference selectexternal connection; and a plurality of multiplexers for selectivelycoupling the voltage reference, the inductive touch sensor externalconnection, and the inductive reference coil external connection to thefrequency mixer.
 11. The integrated circuit device according to claim10, further comprising a frequency divider for dividing a clockfrequency from the clock external connection and applying the dividedclock frequency to the plurality of multiplexers.
 12. An electronicsystem having an inductive touch interface, said system comprising: aninductive touch interface comprising a plurality of inductive touchsensor coils and an inductive reference coil; a first integrated circuitdigital processor; a second integrated circuit inductive touch analogfront end comprising: a voltage reference; a synchronous detector havinga first input coupled to the inductive reference coil; a second inputcoupled to the plurality of inductive touch sensor coils; a third inputcoupled to a reference select signal from the digital processor; afourth input coupled to a clock signal from the digital processor, and afifth input coupled to the voltage reference; a coil driver having anoutput coupled in series with the inductive reference coil and selectedones of the plurality of inductive touch sensor coils, an input coupledthrough an external low-pass filter to a clock output from the digitalprocessor; and an amplifier configured with a low-pass filter and havinginputs coupled to the synchronous detector and an output having voltagevalues representative of inductance values of the plurality of inductivetouch sensor coils and the inductive reference coil, the output of theamplifier is coupled to an analog input of the digital processor;wherein the synchronous detector mixes the clock signal with a signalfrom the inductive reference coil or the at least one inductive touchsensor coil, as selected by the reference select signal, to produce sumand difference mixing products, whereby the amplifier amplifies the sumand difference mixing products and passes the difference mixing productat the output of the amplifier.
 13. The system according to claim 12,wherein the synchronous detector comprises: a decoder having a firstinput coupled to the clock external connection and a second inputcoupled to the reference select external connection; and six analogpass-gates controlled by the decoder, wherein inputs of first and fourthanalog pass-gates are coupled to the voltage reference, inputs of secondand fifth analog pass-gates are coupled to the plurality of inductivetouch sensor coils, inputs of third and sixth analog pass-gates arecoupled to the inductive reference coil; whereby the third and fourthanalog pass-gates are closed when the reference select and clock signalsare at first logic levels, the first and sixth analog pass-gates areclosed when the reference select signal is at the first logic level andthe clock signal is at a second logic level, the second and fourthanalog pass-gates are closed when the reference select signal is at thesecond logic level and the clock signal is at the first logic level, andthe first and fifth analog pass-gates are closed when the referenceselect and clock signals are at the second logic level.
 14. The systemaccording to claim 13, wherein the first logic level is a logic zero(“0”) and the second logic level is a logic one (“1”).
 15. The systemaccording to claim 13, wherein the second logic level is a logic zero(“0”) and the first logic level is a logic one (“1”).
 16. The systemaccording to claim 12, wherein the synchronous detector comprises: afrequency mixer; a decoder having a first input coupled to the clocksignal and a second input coupled to the reference select signal; and aplurality of multiplexers for selectively coupling the voltagereference, the inductive touch sensor signal, and the inductivereference coil signal to the frequency mixer.
 17. The system accordingto claim 16, further comprising a frequency divider for dividing theclock signal and applying the divided clock signal to the plurality ofmultiplexers.
 18. The system according to claim 12, wherein theamplifier is a differential input operational amplifier.
 19. The systemaccording to claim 12, wherein a digital processor determines which oneof the at least one inductive touch sensor coil is selected for voltagemeasurement by the synchronous detector.
 20. The system according toclaim 12, wherein the digital processor is a microcontroller.
 21. Thesystem according to claim 12, wherein the digital processor is selectedfrom the group consisting of a microprocessor, a digital signalprocessor (DSP), an application specific integrated circuit (ASIC), anda programmable logic array (PLA).